Compositional evolution of igneous garnets: calcic garnets from alkaline rocks of Terskiy Coast (Kola Alkaline Carbonatite Province)

The study examined garnets in lamprophyric and carbonatitic dykes and massif melilitolites from the Devonian Kola Alkaline Carbonatite Province (North-Eastern European craton). We analyzed major and trace element compositions of garnet in 14 well-characterized samples of aillikite, carbonatite, alnöites, monchiquites, nephelinite and turjaites and correlated the garnet zoning with the sequence of magmatic crystallization and late deuteric changes. The garnets occur in all textural positions, from early phenocrysts to groundmass phases and deuteric pseudomorphs. The garnets are calcic and classified into five compositional types, 1) high-Zr, 2) high-Ti, 3) medium-Ti, 4) low-Ti, and 5) high-Al. These garnet types combine in distinct ways comprising four zoning patterns (turjaite, nephelinite, monchiquite and carbonate-related) repeated in different rock types. Fractional crystallization controls the observed garnet evolution from high-Ti and high-Zr to garnets poorer in Ti, but richer in Al. Garnets progressing from magmatic to deuteric crystallization evolve from heavy rare earth elements (HREE)-enriched to light rare earth elements (LREE)-enriched and become depleted in trace elements. We interpret high-Zr garnets as antecrysts crystallized from deep alkaline carbonate-rich melts and subsequently destabilized in evolved shallow lamprophyric melts. High-Al garnets have late magmatic or deuteric origin as they replace melilite or carbonate, have low Y/Ho ratio and occur only in a location where carbonatites formed by melt and fluid immiscibility. Significant compositional changes accompanying magmatic crystallization and variations in accessory mineralogy in a single rock type result in a very wide range of major and trace element compositions of garnets and complicate their use as petrogenetic indicators.